This invention relates to the removal of sulfur from coal using a ferric solution. It further relates to a method of monitoring the desulfurization process to prevent the loss of iron from solution by precipitation of insoluble iron compounds.
The combustion of coal which contains sulfur can release sulfur dioxide into the atmosphere and thus represents a potential serious health and environmental problem. It is necessary to remove this sulfur either by post-combustion treatment of the sulfur emissions or by pre-combustion treatment of the coal itself. One of the processes for removing pyritic sulfur (FeS.sub.2) from coal is disclosed in U.S. Pat. No. 3,768,988. This process, commonly called the Myers process, involves the aqueous extraction of sulfur from pyrites in coal using a solution containing a ferric ion. In the process of freeing the sulfur from the pyrite, the iron is chemically reduced and is present in the resulting slurry as ferrous ions. It is economically necessary in such a process to be able to regenerate ferric ions for reuse in the desulfurization process. As disclosed in the patent referred to above, the conventional method for regeneration of ferric ions from ferrous ions is the air oxidation of the ferrous ions back to ferric ions. It has now been found that exposure of the iron ions to oxygen, particularly in this reoxidation process, can result in the loss of a part of the desulfurization reagent and contamination of the coal through the precipitation of iron-containing solids out of the reaction slurry. The problem is especially serious if the coal contains large amounts of basic materials.
It is thus an object of this invention to provide a method of controlling loss of iron ions as precipitated solids in the desulfurization of coal using an iron-containing reagent.
It is a further object to make the desulfurization process at least self-sustaining with respect to ferric ions.
It is a further object of this invention to provide a system for the regulation of the pH of a desulfurization reaction slurry.